Thalassemia Gene Therapy By In Vivo Transduction of Mobilized Hematopoietic Stem Cells (HSCs) with an Integrating Hybrid Adenovirus Vector System

To overcome the cost and complexity of current thalassemia ex vivogene therapy protocols, we developed a minimally invasive and readily translatable approach for in vivo HSC gene delivery which abrogates the need for HSC leukapheresis, CD34+ cell selection, ex vivo HSC culture, myeloablation and ultimately, transplantation. Our approach involves HSC mobilization with G-CSF/AMD3100 andintravenous injections of a hybrid vector system consisting ofa CD46-targeting, helper-dependent adenoviral vector and the hyperactive Sleeping Beauty transposase (SB100x) that mediates integration of thevector-encoded γ-globin and mgmt^P140K genes. Pretreatment with glucocorticoids before virus injectionsis used to blockthe release of pro-inflammatory cytokines andimmunosuppression is applied in order to avoid responses against human g-globin- and MGMT protein-expressing cells. We tested our approach in a mouse model recapitulating the phenotypeof human β-thalassemia intermedia (Hbb^th-3/hCD46⁺⁺ mice). At week 8 post transduction, hCD46^+/+/Hbb^th-3 mice expressed HbF in 31.2±2.7% of circulating erythrocytes. Due to a significant drop in HbF expression by week 16 (11.9±3.0%), a 4-dose O⁶BG/BCNU treatment was administeredin order to in vivo select forgene corrected hematopoietic progenitors, thus recovering the HbF expression in76.0±5.7% of the circulating erythrocytes, by week 29 post in vivo transduction. With an average vector copy number of 1.4/cell, the human γ-globin to mouse α-globin expression was ~10% by HPLC and the human γ-globin to mouse β-globin mRNA ratio ~10%, by qRT-PCR. Hematological parameters (RBCs, Ht, Hb, MCV, RDW, Reticulocytes) at week 29 post in vivo transduction, were significantly improved over baseline or were indistinguishable from normal values, suggesting near to complete phenotypic correction. Treated mice showed significant reduction of spleen size, extramedullary erythropoiesis and parenchymal hemosiderosis. After secondary transplantation and without in vivo selection, more than 90% of donor-derived erythrocytes (CD46+) were g-globin-positive, up to 20 weeks post-transplant. Safety was demonstrated by the good tolerability of treatment, the absence of alterations in hematopoiesis, the normal colony-forming potential of bone marrow cells and the random integration pattern of our vector system. Overall, we present a simplified platform for gene therapy of thalassemia, which can serve as a cost-efficient and "portable" approach to make gene therapy accessible even to resource-poor regions where thalassemia major is endemic but only minimally complex strategies could be adopted.